Those who are avid readers of ingredient labels will have often seen the word lecithin mentioned. These lecithins are important in food chemistry. It’s a very handy emulsifier which means it helps solubilise lipid or oil favouring agents such as flavours, aroma chemicals and the like in foodstuffs. It also helps to encourage the dispersion of powders when they are added to water by improving the so called wettability factor and to reduce clumping. Bakers add it to dough for improvement and maintain freshness without staling. It’s a common component of non-stick cooking spray and of natural lubricants, such as reducing cheese sticking together.
Composition Of Lecithins
Lecithin is a mix of natural materials, the phospholipids, glycolipids, neutral lipids and sugars which are part of the membranes of living cells.
Egg yolk is a prime source in pharmaceutical applications and parenteral nutrition whilst various seeds such as sunflower and some oily fish such as sardines provide the bulk for food use. It is in fact found in all animal and plant tissues which demonstrates its ubiquitous distribution.
The French chemist Theodore Gobley first isolated lecithin in 1846 but only named it as such in 1850. Lecithins are relatively soluble in water and generally insoluble in hexane, diethyl ether, chloroform, ethanol and acetone which constitutes an important step in their extraction and isolation. The industrial process relies on aqueous degumming the oil once it is extracted from seeds with acetone then precipitating it. Supercritical fluid extraction using carbon dioxide is preferred from an environmental perspective.
The major phospholipids (PLs) are phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidylinositol (PI) and phosphatidic acid (PA). Phospholipids are like diglycerides and have a lipophilic portion which is two fatty acids esterified to a glycerol backbone with affinity for oil. They also have a polar, hydrophilic or water-loving portion which is based on a phosphate ester, commonly linked to choline, ethanolamine or inositol. The content and ratio of each lecithin characterises the lecithin. Lecithin properties are due in part to their zwitterionic nature. They carry both positive and negative charge which accounts in part for their emulsifying power.
As an ingredient, lecithin is available in many formats:-
(a) an unmodified liquid,
(b) chemical and enzymatically modified forms
(c) a de-oiled powder.
The powdered variants, occur as a fine powder or are granulated, but need much of their oil portion removed to ensure water-dispersibility. De-oiled lecithin is almost flavourless which is very handy for product development.
The Production Of Lecithins
Lecithins are a by-product of processing crude soybean oil or crude sunflower oil. During processing of these vegetable oils, a gum is generated that has to be removed in the degumming step of oil refining. The crude gum is treated and purified to produce various commercial forms of lecithin.
Crude soybean oil contains 2% lecithin whilst corn and cotton seed oils contains around 1%. Only soybean oil derived lecithin is commercially profitable to produce because soybean oil is produced in such large quantities. Much of the excess gum from other sources is returned as animal feed especially as a way of fattening up animals for market (see below).
Egg yolks contain 2% phospholipids which is why egg is a such an excellent emulsifier when used in sauces and mayonnaise. Nowadays, egg yolk lecithin is too expensive to produce but it is still valued when added as part of egg in cakes and other baked goods.
The crude gum from soy is dehydrated to help with lecithin extraction. The insoluble fines are removed by filtration. This dehydrated gum is brown to chocolate brown in colour and is formed by caramelization reactions during heat processing. These browning products are derived from pigments in the soybean.
To reduce the brown colour, the gum is bleached to produce a more acceptable light brown colour. Bleaching agents include 1.5% hydrogen peroxide which produces a single-bleached lecithin. A further addition of 0.5% benzoyl peroxide produces what is called double-bleached lecithin. Further types of lecithin are generated by reacting with more hydrogen peroxide and some lactic acid. This leads to hyroxylation of the unsaturated fatty acid side-chains at the double bond. The product is a dihydroxystearic acid if the source is oleic acid. Hydroxylated lecithins are more dispersible in cold water than other types of lecithins which is why there it is produced on a large-scale. Hydroxylated lecithins are more effective as emulsifiers in oil and water emulsions.
HBL and % AI
Lecithin properties are characterised by (a) the acetone or occasionally hexane insoluble content (% AI) and (b) a hydrophilic/lipophilic balance (HLB). The acetone insoluble value, or its AI, describes the amount of active componentry in the lecithin. It is the percentage of polar material such as phospholipids, glycolipids and sugars, contained in its molecule. The typical AI for standard fluid lecithin is 62% to 64%, while de-oiled lecithin contains a minimum 97% AI.
The hydropholic/lipophilic balance (HLB) is a measure of the preference of the emulsifier for oil or water. The higher the HLB, the more hydrophilic or water-loving the emulsifier. The HLB values reflect the size and strength of the hydrophilic and the lipophilic (nonpolar) groups in the lecithin. Most of the surfactant properties of lecithin can be attributed to the phospholipids. The HLB of standard fluid lecithin is 2 to 4, while that of modified fluids range from 6 to 12. The HLB range of de-oiled lecithin is 7 to 10. Lecithins with an HLB below 6 are not readily water-dispersible. This has important implications for oil-in-water emulsions and food systems where water is a key component.
By reducing the surface tension between immiscible liquids, lecithin promotes the formation of stable oil-in-water and water-in-oil emulsions. Standard-grade fluid lecithins are blends of natural phospholipids and soybean or sunflower oil. Unbleached and bleached versions are available with an AI range from 62% to 72% with an HLB of 2 to 4. Standard-grade lecithin is widely used in margarine, a water-in-oil emulsion. Value-added lecithins have been modified and further processed to improve water dispersibility and functionality for oil-in-water emulsions.
Enzyme Modified Lecithins
Enzyming the lecithin improves its functional performance. Phospolipase A2 removes the fatty acid from the middle carbon (sn-2 position) of the glycerol portion of the molecule. This produces a structure that resembles a typical monoglyceride, but with a more polar head group due to the phosphate. Enzyme-modified lecithins have an HLB of 8 and are available in both fluid and de-oiled forms. Hydroxylated lecithins are chemically modified and have an HLB of 10. They work well in difficult emulsions, such as pourable salad dressings. By adding complementary surfactants, such as ethoxylated monoglycerides, to lecithin, highly water-dispersible lecithins have an HLB as high as 12.
Wettability & Dispersion
Lecithin improves powder dispersion, rehydration, wettability and instantising. It is ideal for instant foods and juices. It is usually added to a powder through mixing, added during agglomeration or to the liquid feed when spray drying. Creamers that need to disperse or instantise rapidly usually rely on high-HLB lecithins. Otherwise, these have a very low affinity for water and are very slow to wet and disperse. For powders that wet too fast on the surface and form “fish eye” lumps that are wet on the outside and dry on the inside, a lower-HLB lecithin is recommended to slow down and control the wetting. The typical level of use for instantising is 0.5% to 2% lecithin, based on the powder weight. Low-viscosity lecithins can be sprayed directly without heating or dilution.
Lecithin also reduces static to effectively wet dusty particles, and thus alleviate dusting as in custard powders. Lecithin, added to 0.25%-0.5% w/w to a very dusty powder can dramatically reduce product loss, reduce sanitation concerns from stray powder and eliminate potential damage to air-handling systems.
Reducing Stickiness With Lecithins
Lecithin creates an even, non-stick coating to prevent foods from sticking to one another and to contact surfaces. As such, lecithin alleviates a variety of manufacturing issues, mainly increased rework, losses of products and increased downtime for cleaning. Lecithin is the functional ingredient in consumer aerosol cooking sprays, as well as commercial release products for pans and oven belts. Using a heat-resistant lecithin greatly reduces the potential for non-enzymatic browning in high-heat applications.
Several options exist for conveyoring and oven-belt release. Low-viscosity lecithin can be applied as is, either sprayed onto metal or plastic conveyor belts or is added to a dip tank as the belt travels through. High-HLB lecithin can be mixed with water at a level of 5% to 15% and added to the dip tank.
Lecithin is added to baking products and mixes from 0.1% to 1 % w/w on a flour basis or baker’s percent. It serves as an emulsifier, stabiliser, conditioner and release agent. In the yeast-raised doughs, it improves moisture absorption, fermentation tolerance, the shortening value of fat and shelf-life. Lecithin can improve machinability, tenderness and crumb structure, and extend shelf life by delaying staling. A higher-HLB, water-dispersible lecithin, such as a de-oiled or enzyme-modified variant, produces some of the best results, because water is a key ingredient in forming doughs and batters.
It is a common ingredient of many animal feeds (ca. 1-3% w/w) milk replacement formulas for calves and veal, poultry feed and other foodstuffs. It helps solubilise fats in chocolate, for example minimising bloom formation, helping to minimise viscosity and solids inconsistencies, distributing fats in fudge, caramels, nougats etc. It fixes flavours. In other dried goods, it acts as a release agent. It is claimed to have antioxidant properties and potentiate those of other antioxidants.
Lecithin is approved by the United States Food and Drug Administration for human consumption with the status “generally recognized as safe” i.e. GRAS. Lecithin is under EU regulation, a food additive, designated as E322 but often just labelled as ‘lecithin’ with the source added for allergenic purposes. Soy is a notifiable allergen.